The present invention relates to cellular communication systems, and more particularly to the operation of relay nodes in a cellular communication system.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. As the mobile terminal moves from one cell to another, the network hands over responsibility for serving the mobile terminal from the presently-serving cell to the “new” cell. In this way, the user of the mobile terminal experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
Present-day cellular communication systems are typically based on a homogenous network, mainly consisting of large macro cells, each cell having one transmitter/radio unit that serves the entire cell. In future cellular systems, heterogeneous network architectures can be expected comprising a mix of large macro and small pico/femto cells. Furthermore, there will also be situations where a specific cell has several radio units. Such solutions make it possible to utilize advanced multiple-input-multiple-output (MIMO) technology and beam forming schemes and thereby improve the entire system spectral efficiency.
Relay Nodes (RNs) and repeaters are also sometimes deployed in a cellular communication system to give increased coverage without having to install another serving transceiver station (e.g., a base station). In the system known as the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), for example, an RN is intended to give increased coverage without the need to install yet another Evolved nodeB (eNB). In the down-link (DL) (i.e., the direction from eNB to the user equipment, “UE”), the RN receives the data from the eNB, decodes it, re-encodes the decoded data, and then transmits the re-encoded data to the UE. In the uplink (UL) (i.e., the direction from UE to the eNB) the corresponding procedure is done, but in the other direction instead. Although the output power of signals transmitted by a RN can be similar to those transmitted by an eNB, it is envisioned that there will be many deployments in which a significantly lower output power will suffice. When a RN is used, the eNB is referred to as a donor eNB (DeNB). The link between the DeNB and the RN is referred to as the backhaul link and commonly denoted Un, whereas the link between the RN and the UE is referred to as the access link and commonly denoted Uu.
Since a RN has two transceivers, one for each link, some care has to be taken when operating these links in order to ensure that they do not interfere with one another. In conventional systems, there are two principally different possibilities to achieve this interference avoidance. In a first alternative, the two links are caused to use different frequency bands (out-of-band relaying), in which case the coexistence between the two links is ensured by means of filtering. In a second alternative, the two links use the same frequency band (in-band relaying) but are caused to use different time-slots in a frame so that coexistence is ensured by means of scheduling.
Repeaters have similar functionality as RNs. However, the two types of devices are distinguishable from one another. One distinction is that a repeater does not decode the data and then re-encode the date that it receives from either the DeNB or UE, but rather only amplifies and then retransmits the received signal. The functionality of what is herein referred to as a “relay” is therefore commonly referred to in the art as “Decode and Forward (DF) relaying”, whereas the functionality of what is herein referred to as a repeater is commonly referred to in the art as “Amplify and Forward (AF) relaying”.
A repeater is often faced with the problem of having to receive a rather weak signal at the same time that it is transmitting a signal that is considerably stronger. While the power of the received signal might be on the order of −80 dBm, the power of the transmitted signal might be on the order of 0 dBm. In order to avoid self-oscillation, this puts rather hard requirements on the amount of isolation required between transmission and reception. As a rule-of-thumb, the isolation should be about 10 dB higher than the amplification of the signal. For instance, if the amplification is 80 dB, as would be the case for the example above, then the isolation should be 90 dB.
There is a fundamental difference between a repeater and a relay when it comes to the requirements on self-interference. Since a repeater does not decode the information, the requirements will be set by the quality of the transmitted (amplified) signal. For a relay, on the other hand, the received signal needs to be decoded, which means that the requirements will be determined by the requirements of the receiver in the relay.
A major problem with the existing solutions for a RN is that the two links, Un and Uu, either need to be coordinated in time, or else have to be allocated different frequencies. The former implies restrictions on the maximum data rate that can be supported as well as on the scheduling. The latter implies that twice the amount of spectrum is needed. The inventors of the subject matter described herein are aware that under certain conditions, it might be feasible for a RN to transmit on Un while receiving on Uu (and vice versa). However, this requires some means for effectively suppressing the generated interference from transmitter to receiver.
For at least the foregoing reasons, it is desirable to have improved apparatuses and methodology for performing relay functionality.